docs: def ipv4_is_defined(address):
      """
      The function for checking if an IPv4 address is defined (does not need to
      be resolved).
  
      Args:
          address (:obj:`str`): An IPv4 address.
  
      Returns:
          namedtuple:
  
          :is_defined (bool): True if given address is defined, otherwise
              False
          :ietf_name (str): IETF assignment name if given address is
              defined, otherwise ''
          :ietf_rfc (str): IETF assignment RFC if given address is defined,
              otherwise ''
      """
  
      # Initialize the IP address object.
      query_ip = IPv4Address(str(address))
  
      # Initialize the results named tuple
      results = namedtuple('ipv4_is_defined_results', 'is_defined, ietf_name, '
                                                      'ietf_rfc')
  
      # This Network
      if query_ip in IPv4Network('0.0.0.0/8'):
  
          return results(True, 'This Network', 'RFC 1122, Section 3.2.1.3')
  
      # Loopback
      elif query_ip.is_loopback:
  
          return results(True, 'Loopback', 'RFC 1122, Section 3.2.1.3')
  
      # Link Local
      elif query_ip.is_link_local:
  
          return results(True, 'Link Local', 'RFC 3927')
  
      # IETF Protocol Assignments
      elif query_ip in IPv4Network('192.0.0.0/24'):
  
          return results(True, 'IETF Protocol Assignments', 'RFC 5736')
  
      # TEST-NET-1
      elif query_ip in IPv4Network('192.0.2.0/24'):
  
          return results(True, 'TEST-NET-1', 'RFC 5737')
  
      # 6to4 Relay Anycast
      elif query_ip in IPv4Network('192.88.99.0/24'):
  
          return results(True, '6to4 Relay Anycast', 'RFC 3068')
  
      # Network Interconnect Device Benchmark Testing
      elif query_ip in IPv4Network('198.18.0.0/15'):
  
          return (results(True,
                  'Network Interconnect Device Benchmark Testing',
                          'RFC 2544'))
  
      # TEST-NET-2
      elif query_ip in IPv4Network('198.51.100.0/24'):
  
          return results(True, 'TEST-NET-2', 'RFC 5737')
  
      # TEST-NET-3
      elif query_ip in IPv4Network('203.0.113.0/24'):
  
          return results(True, 'TEST-NET-3', 'RFC 5737')
  
      # Multicast
      elif query_ip.is_multicast:
  
          return results(True, 'Multicast', 'RFC 3171')
  
      # Limited Broadcast
      elif query_ip in IPv4Network('255.255.255.255/32'):
  
          return results(True, 'Limited Broadcast', 'RFC 919, Section 7')
  
      # Private-Use Networks
      elif query_ip.is_private:
  
          return results(True, 'Private-Use Networks', 'RFC 1918')
  
      # New IANA Reserved
      # TODO: Someone needs to find the RFC for this
      elif query_ip in IPv4Network('198.97.38.0/24'):
  
          return results(True, 'IANA Reserved', '')
  
      return results(False, '', '')